Lesson Four: Say, "Yes."
This is a basic GMing rule anyway, but feel comfortable saying, "Yes," to your players. If your scenario goals rely on your players NOT being able to do things, your scenario is probably broken.
Someday, I will learn this lesson. I had a pretty good scenario going with a game a couple years ago, and there I was... the player wanted to do something, and I said, "Um, you can't." Now, canonical game mythology and rules were with me... in fact, it was one of the base details of the game's premise. There were several other methods she could have used to follow the NPC speeding away, but that wasn't one of them.
And it killed the scenario.
I mean, I had other players backing me up, but it was a sour note that poisoned the symphony. I didn't quite yell, "Go into any other game on the floor and ask them if you can do this, and they'll shout, `NO!'" but I was filled with the irrational impulse to do so, which somewhat killed my mood, and just re-iterated my feeling that since I had to say, "No," I had failed the player, too.
There's a rule in the Amber RPG1 that basically says, "The only reason a character fails is if there's someone stopping them." For a worldview where you can manipulate probabilty to ridiculous degrees2, there really does need to be a face behind the mask of the locked door. The universe rarely says, "No," so if you get a, "No," it needs to be personal.
That above situation? Amber game. And if the player got a, "No," they should have been asking, "Who is this person who is fighting me on this?"
It should never be the GM. I've been lucky, but I know of many a game where the "game" is the confrontation between GameMaster and player wits.3 Don't get me wrong, I love being right (especially about puzzles) but I don't like the conflict and stress and, finally, utter uselessness of fighting a GM. The GM will always win. I want to be the GM who shares the excitement of the players' "winning."
The LintKing loves Earthdawn. Sure, I will run it, which isn't the point of contention between us. The reason I'm not running it is that I say, "No," to a couple of pieces of the game. One of which is a whole player race. (Windlings.) I think that I have a great reason, which is that I don't really want to have to consider the z-axis. They can fly.4 In a game of traps and puzzles in dungeons, that's a bit of a deal-breaker for me. Doesn't matter that they can't fly all that well - it still means I have to do extra work to find traps that simply having a gnat as a familiar won't break. (OK, that's a bit of hyperbole, but still...)
And that's definitely the wrong attitude to have. Not that I'm expecting the LintKing to just whip out a windling nethermancer from the Blood Wood with a Horror familiar as a character5, but why am I worried that the puzzles will be too easy? Didn't I learn Lesson #3? Because really, when you think they're easy, the players generally step in their own way long enough to make it a reasonable challenge.6
Now, it doesn't mean, "Allow your principles to be broken." I have said, "Yes," to character concepts and ideas that really have broken some of the ideas behind a game, and I regret it. A well spoken, "No," is a powerful tool. A spoken "No," in haste or in just, "I don't want to have to deal with this now," however, is (like the others in this series) a Con Game Breaker.
1 It's diceless, not rule-less.
2 Moreso in the game than the books, so I try to strike a bit of a balance.
3 My dad got me a button saying that I, as GM, had more hit points than the players could imagine. Dad was old school, and I was raised old school, but I never wore it because I didn't approve of the sentiment.
4 The fact that their base racial personality is that of many small opportunistic creatures doesn't help their case.
5 Except he totally would. And the Horror would be in an obsidiman body.
6 We've had discussions about the characters of NCIS as a dungeon-crawling team, and I've explained the reason we don't do things like that is because they simply demolish the dungeons. (What a smart group does in part, anyway.)